This invention relates to a battery charger having three different charging rates. A first "trickle" charge rate is used, if needed, to bring the battery up to a predetermined minimum charge capacity without overheating. Once the battery has reached this minimum charge level without overheating, it is presumed safe to use a high rate charger on the battery so as to minimize the overall charging time of the battery. This high rate charge must be terminated as the battery reaches full charge, due to known dangers of high rate charging on a fully charged battery. To bring the battery up to and/or to maintain its full charge, a lower "tickle" charge rate is employed. During the time the battery is rapid charging, this battery charger continually checks certain parameters of the battery to be sure that it is still within safe ranges for high rate charging.
Batteries store energy in chemical form. When a load is placed across the battery, a chemical reaction within the battery produces the required electric current. A normal battery of "throw away" type has chemicals which react to produce electricity, but are generally not subsequently reusable. Rechargeable batteries, on the other hand, contain chemicals which can undergo reversible reactions, that is to say undergo a reaction in one direction to produce electricity, and upon application of an electric charge to the battery, undergo a chemical reaction in the reverse direction thus storing the energy for later use.
There are many popular kinds of rechargeable batteries. The most popular presently are the nickel cadmium and lead acid types. The battery charger disclosed in the present invention is optimized for use with a nickel cadmium type battery.
Nickel cadmium batteries suffer from many problems during charging. It is extremely unsafe to overcharge a nickel cadmium battery as this can result in severe internal damage to the battery or even explosion. By applying too much charge to a battery that is already fully charged, the energy is dissipated by a temperature rise which leads to a phenomenon known as "gassing". This gassing is produced by chemicals within the battery absorbing the energy applied and thus changing into a gaseous state. Since a nickel cadmium battery is typically hermetically sealed, this production of gas can lead to pressure within the battery which, in extreme cases, could result in explosion. Gassing can also result from attempting to charge a battery that is at too high or too low a temperature. Another problem that can be encountered during charging is that when a battery gets to a certain temperature during normal charging it can lead to a condition known as "thermal runaway", wherein a majority of the energy being applied to the battery results in a temperature increase.
It also is desirable to charge a battery as rapidly as possible. However, the higher charge rates make the above mentioned effects more likely to occur. In the prior art this tradeoff was often accommodated by providing a two rate charger. In general, a medium rate charge was applied to bring a battery nearly up to its nominal voltage. At that point, the rate would be changed to a very low rate which was intended to bring the battery up to full charge and to keep it fully charged.
One such two rate charger is described in U.S. Pat. No. 3,553,561 to Lesher. Lesher describes a two rate charger for use within an emergency lighting system wherein a high rate charge is used to bring the battery up to a cutoff point just below the gasing point of the battery, and a charge of relatively low current thereafter is used to maintain the battery at this charge.
U.S. Pat. No. 3,624,481 to Macharg also describes a two rate charger for nickel cadmium batteries which tests battery voltage against a reference voltage to determine when the current rate should be altered. U.S. Pat. No. 4,386,308 to Emile, Jr. et al teaches a two rate battery charger wherein hysteresis is used to prevent undue oscillating between different rate charges at the switching point. U.S. Pat. No. 3,855,516 to Fairchild discloses a battery charger which charges at a high rate for a predetermined time period after which a trickle level is initiated. Other similar chargers are disclosed in U.S. Pat. No. 3,603,862 to Chase et al, U.S. Pat. No. 3,660,748 to Clayton et al, U.S. Pat. No. 4,134,056 to Fukui et al, and U.S. Pat. No. 3,863,129 to Yamauchi et al.
An example of a fast charger for a nickel cadmium battery is already described in U.S. Pat. No. 4,468,605 to Fitzgerald et al. This battery charging device also monitors battery temperature and voltage to determine the safety of applying a high rate charge to the battery. However, even Fitzgerald teaches only a two rate battery charger. Instead of initially slowly charging the battery to a voltage at which it is safe to apply a high rate charge, Fitzgerald pulses the battery at a low duty cycle with a high rate charge, and determines the approximate first derivative of battery voltage during this charging period. Using a complicated algorithm, this battery charger then determines whether the battery is in a safe operating region to be high rate charged. If the battery is in such a safe region, a high rate charging cycle begins. When a certain voltage has been reached the high rate charging cycle is terminated and a trickle charge begins. The disclosure of Fitzgerald et al is hereby expressly incorporated herein by reference.
Conspicuously absent from all of these prior art documents is any suggestion of using a third charging level as now disclosed in this application.
A problem existing even in prior two-rate chargers is that a battery can not safely be rapid charged if it is in sufficiently poor condition (e.g., extremely low open circuit voltage, lack of continuity, etc.). The problems and dangers discussed above could have extreme consequences if an extremely high charge were used during the time these poor conditions persist. Thus, devices in the prior art typically used only an intermediate charging rate as the "high" charge rate.
Another problem with prior two-rate chargers is that, in reality, a somewhat higher rate often is needed initially to bring the battery up to a safe voltage for rapid charge than is needed merely to sustain battery charge once the high rate charging is complete. Thus, a two rate charger typically employs compromise charge rates between the optimal rates. However, with such a compromise, a severly discharged battery might never be brought to safe conditions for applying rapid rate by this less than optimal initial rate of charge--and once the battery is fully charged, any higher than necessary charging current will dissipate its energy in the form of heat. As discussed above, heating of these batteries can lead to serious side effects, and also shortens the battery's life, and makes it more subject to failure.
The present invention initiates battery charging at a medium range rate ("trickle"), while it is charged to a predetermined voltage. The purpose of this initial medium range charge is to insure safe conditions before applying a truly "high" charge rate, by test of the battery's condition. During this initial period, the temperature of the battery is also continually being monitored by a microprocessor controller. When the battery reaches a certain voltage without excessive overheating, it will be presumed safe to apply a rapid "high" charge to the battery. This rapid charge will be applied to the battery until such time as the battery reaches a voltage very close to its nominal capacity. At this time, a third charging rate is applied, a very low "tickle" charge.
Other objects, features, and advantages of the present invention will become more apparent upon consideration of the following detailed description with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures.